docs: def parse_mpi(s):
      """See https://tools.ietf.org/html/rfc4880#section-3.2 for details."""
      bits = s.readfmt('>H')
      blob = bytearray(s.read(int((bits + 7) // 8)))
      return sum(v << (8 * i) for i, v in enumerate(reversed(blob)))